making changes is more cumbersome than in their server counterparts. In Chapter 1 we
explained that although the control plane of legacy networks had sophisticated ways of autonomously
and dynamically distributing layer two and layer three states, no corresponding protocols exist for distributing the policies that are used in policy-based routing. Thus, configuring security policy, such as
ACLs or virtualization policy such as to which VLAN a host belongs, remains static and manual in
traditional networks. Therefore, the task of reconfiguring a network in a modern data center does not
take minutes but rather days. Such inflexible networks are hindering IT administrators in their attempts
to automate and streamline their virtualized data center en vironments. SDN holds the promise that the
time required for such network reconfiguration be reduced to the order of minutes, such as is already
the case for reconfiguration of VMs.